1. Field of the Invention
The present invention relates to a liquid crystal display device for use in a television set, a personal computer, a word processor, an OA equipment, or the like.
2. Description of the Related Art
A matrix-type liquid crystal display device is known in the art. As schematically illustrate in FIG. 8, the liquid crystal display device includes a matrix substrate 28, a counter substrate 29, a liquid crystal layer 27 interposed between the substrates 28 and 29, and a pair of polarizing plates 30 respectively provided on the substrates 28 and 29. The liquid crystal display device further includes a light source (i.e., a back light) 31 on the rear side of the device, so that the device functions as an optical shutter.
FIG. 9 is a plan view illustrating the matrix substrate 28. FIG. 10 is a plan view illustrating the counter substrate 29 viewed from the side of the matrix substrate 28.
The matrix substrate 28 includes gate lines 32 and source lines 33 crossing each other. A thin film transistor 34 as a switching element is provided in the vicinity of each intersection of the gate line 32 and the source line 33. Pixel electrodes 35 are provided in a matrix and each connected to the gate line 32 and the source line 33 via the thin film transistor 34. On the other hand, the counter substrate 29 includes a light blocking film 37 and a color filter (not shown). The light blocking film 37 includes openings 36 respectively corresponding to the pixel electrodes 35 on the matrix electrode 28. The counter substrate 29 further includes counter electrodes 38 disposed on the entire surface thereof. An alignment film (not shown) is provided on each of the matrix substrate 28 and the counter electrodes 29. The alignment film is subjected to a rubbing treatment so that liquid crystal molecules are aligned in a desired direction.
In the liquid crystal display device having such a structure, a voltage to be applied across each pixel through a corresponding portion of the liquid crystal layer 27 can be controlled by inputting an image signal to the corresponding pixel electrode 35 via the corresponding thin film transistor 34. When a voltage is applied across a portion of the liquid crystal layer 27, the orientation of the liquid crystal molecules in that portion of the liquid crystal layer 27 changes depending upon a dielectric anisotropy of the liquid crystal molecules.
Specifically, in a vertical orientation mode, the liquid crystal molecules are oriented in a substantially vertical direction in the absence of an applied voltage, and the liquid crystal molecules are inclined in a substantially horizontal direction in the presence of an applied voltage, using a liquid crystal material having a negative dielectric anisotropy. In order to have an uniform inclination of the liquid crystal molecules, the alignment film is subjected to a rubbing treatment. Thus, the liquid crystal molecules are inclined in a rubbing direction 40 in the presence of an applied voltage. The pair of polarizing plates 30 are arranged so that the respective absorption axes are at about 90.degree. with respect to each other and at about 45.degree. with respect to the rubbing direction 40.
The principle of light control of the above-explained liquid crystal display device is as follows.
Light from the back light 31 is polarized into linearly-polarized light by the polarizing plate 30 on the rear side (incident side) prior to being incident on the liquid crystal layer 27. In the absence of an applied voltage, the liquid crystal molecules are in a vertical orientation and have no phase difference. Thus, the liquid crystal layer 27 transmits the linearly-polarized light therethrough without changing its polarization axis so as to allow it to be incident upon the other polarizing plate 30 on the front side (output side). The perpendicularly arranged polarizing plate 30 on the front side absorbs the linearly-polarized light incident thereupon, thereby providing a black display.
In the presence of an applied voltage, on the other hand, the liquid crystal molecules are inclined at about 45.degree. with respect to the polarization axis of the linearly-polarized light, which has passed through the polarizing plate 30 on the rear side, thereby providing a birefringence. Thus, the linearly-polarized light incident upon the liquid crystal layer 27 becomes elliptically-polarized light, circularly-polarized light, or linearly-polarized light whose polarization axis is shifted by about 90.degree.. The polarizing plate 30 on the front side absorbs a component of the incident light along the absorption axis thereof, thereby changing the light transmittance of the device. The transmittance T in such a situation can be represented as follows: EQU T=sin.sup.2 (.delta./2) EQU .delta.=(2.pi./.lambda.).times..DELTA.n.times.d.
Thus, a gray-scale display and a white display are realized. In the above expressions, .DELTA.n denotes an apparent refractive index anisotropy of the liquid crystal, and d denotes a cell gap.
The apparent refractive index anisotropy is a refractive index anisotropy of the liquid crystal molecules in a plane parallel to the substrate surface (i.e., perpendicular to the light transmitting direction), and increases as the liquid crystal molecules are inclined toward the horizontal direction as the applied voltage increases. Therefore, it is possible to continuously change the light transmittance by changing the value of the applied voltage.
The above-described vertical orientation mode has a higher response speed than those of a TN (twisted nematic) mode or an STN (super twisted nematic) mode which are commonly employed, since the vertical orientation mode does not employ a twisted structure in the liquid crystal layer. Moreover, in the vertical orientation mode, a black display is obtained by applying no voltage, whereby there is no birefringence and no phase difference. Thus, light can be blocked by the pair of polarizing plates perpendicularly arranged to each other. Therefore, a high contrast can be easily obtained.
The liquid crystal display device as described above is advantageous in terms of the response speed and the contrast. However, it has a difficulty in uniformly controlling the orientation of the liquid crystal molecules in the presence of an applied voltage.
When an image displayed on such a conventional liquid crystal display device is observed, some "fuzziness" or "unevenness" may be recognized, indicating a poor display quality. When each pixel is further observed in detail with a loupe, display non-uniformity may be recognized as schematically illustrated in FIG. 11.
FIG. 11 illustrates liquid crystal molecules inclined radially about point A in the presence of an applied voltage, causing the display non-uniformity. It is believed that such display non-uniformity is observed due to a transverse electric field (an electric field having a component in a direction along the substrate surface) between the pixel electrode and the gate line or between the pixel electrode and the source line. This will be discussed below.
In the above-described liquid crystal display device, in the absence of an applied voltage, liquid crystal molecules 41 are generally oriented in a direction substantially perpendicular to the substrate surface, as illustrated in FIG. 12. Although the liquid crystal molecules 41 are slightly tilted along the rubbing direction 40 as a result of the rubbing treatment, this initial tilting angle is less than 10.degree.. Thus, the generated phase difference is negligible with hardly any influence on the display quality.
In the presence of an applied voltage, e.g., when an electric field perpendicular to the substrate surface is created, the liquid crystal molecules 41 are inclined in a direction in which they are previously tilted (counterclockwise in FIG. 12).
However, in an actual liquid crystal display device, since the pixel electrode 35 is surrounded by peripheral electrode members 42 (e.g., a gate line, a source line, or the like), there occurs a transverse electric field 43, which is not perpendicular to the substrate surface, between the pixel electrode 35 and the peripheral electrode member 42. Accordingly, a moment acts upon the liquid crystal molecules 41 to rotate them in a direction perpendicular to the direction of the electric field. As a result, in region A in FIG. 13 over the pixel electrode 35, a moment acts upon the liquid crystal molecules 41 to rotate them in a counter-clockwise direction due to the initial inclination. In region B in the peripheral region of the pixel electrode 35, a moment acts upon the liquid crystal molecules 41 to rotate them in a clockwise direction.
The direction in which the liquid crystal molecules 41 are inclined is determined by the moment acting upon the liquid crystal molecules 41 and an elastic action between the adjacent liquid crystal molecules 41. In an actual liquid crystal display device, the liquid crystal molecules 41 in region B are at a large angle with respect to the direction of the electric field, whereas the liquid crystal molecules 41 in region A are at a small angle with respect to the direction of the electric field. Therefore, the moment acting upon the liquid crystal molecules 41 in region B is dominant. As a result, the liquid crystal molecules 41 rotate in a clockwise direction in both regions A and B.
Such a transverse electric field as described above occurs on each of the four sides around the pixel electrode 35. Thus, the liquid crystal molecules 41 are inclined toward an internal point within the pixel electrode 35, resulting in a radial distribution in the inclination direction about the internal point, which causes display non-uniformity.
Moreover, the respective internal points about which the distribution occurs are not positioned at positions corresponding to each other among the different pixel electrodes 35, whereby fuzziness is recognized when the entire screen is observed.